We study the rare τ− → π−η(′)ντ decays. These processes occur via isospin violation and belong to the so-called second-class currents unseen in Nature so far. Our analysis is based on the framework of resonance chiral theory supplemented by dispersion relations. In this contribution we discuss the prospects for their discovery at forthcoming B-factories such Belle-II. While we find a total branching fraction for the π−η decay mode of ~1.7 × 10−5, well within the reach of Belle-II, the π−η′ channel might be one or two orders of magnitude more suppressed.

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